The present invention relates to systems for improving the perceived quality of an acoustic signal in the presence of ambient noise. More specifically, the invention relates to systems for automatically adjusting the gain and the compression of an audio signal for use in audio reproduction systems which use a recorded or transmitted audio signal to generate an audible acoustic signal.
As is well known, human perception of acoustic signals, such as music signals generated by an audio reproduction system, is affected by the presence of ambient noise. Therefore, audio reproduction systems designed to operate in noisy environments, such as inside an automobile, often contain components which attempt to compensate for the degradation caused by noise.
Ambient noise in automobiles, caused for example by the engine or the interaction of the tires and the road surface, is usually concentrated below approximately 200 Hz. Such low frequency ambient noise causes at least three distinct types of problems for automotive audio reproduction systems. First, the volume perceived by a listener, commonly referred to as the "apparent volume", is a function of the noise such that the apparent volume decreases as the noise increases. Listeners may wish to maintain the apparent volume at a constant level, but this is difficult since the ambient noise changes dynamically in an automobile (e.g., with changing road conditions and changing vehicle speeds). This problem is referred to as the "apparent volume problem". Second, since the noise is concentrated in the low frequency range, acoustic signals in automobiles are masked more in the lower frequency ranges than in the higher frequency ranges thus causing the signals to lose their desired tone. This problem is referred to as the "uneven masking problem". Third, the presence of ambient noise reduces the dynamic range of acoustic signals which a listener can perceive. Thus, during relatively high ambient noise conditions, soft passages of music may be entirely masked by the noise, and efforts to increase the volume setting to make the soft passages audible (e.g., by turning up the volume control knob) may make louder passages too loud for listener comfort. So in the presence of noise, ideally, desired acoustic signals are compressed to permit a listener to comfortably hear the entire signal. This problem is referred to as the "dynamic range problem".
One prior art method of compensating for the apparent volume problem, referred to as the "noise-only-method", involves increasing the gain of the audio signal as a function of the ambient noise. This method prevents soft passages from being overwhelmed by the noise. However, the method is disadvantageous because it varies the gain irrespective of the volume level set by the user, so the method increases the gain in response to increasing noise even for very high volume levels which may result in producing signals which are painfully loud for a listener, harmful to the audio reproduction equipment, or both.
U.S. Pat. No. 4,479,237 teaches a system for compensating for the apparent volume problem which varies the gain `c` of the audio signal according to the relation shown in Equation (1) ##EQU1## in which `n` represents the noise power level, `a` is the manually set volume level (i.e., `a` is determined by the position of the volume control knob), and k.sub.1, and k.sub.2 are constants. This system is an improvement over the noise-only-method because when the manually set volume level is high (i.e., when the k.sub.1 -a/k.sub.2 ! term is relatively small), the system only increases the gain by a relatively small amount in response to increasing noise and thus prevents the system from generating dangerously high signals. Further, when the manually set volume level is low (i.e., when the k.sub.1 -a/k.sub.2 ! term is relatively large) the system significantly increases the gain in response to increasing noise levels and thus prevents increasing noise from overly decreasing the apparent volume. However, this system is disadvantageous because the simple linear function of Equation (1) does not adjust the gain aggressively enough to maintain the apparent volume at an optimal level. Further, this system does nothing to address the uneven masking problem or the dynamic range problem.
A well known approach to compensating for the dynamic range problem involves compressing the audio signal by increasing, or boosting, the gain of the audio signal when the level of the audio signal is relatively small (so that signals near the noise floor are boosted above the floor) and by maintaining the gain of the audio signal at a constant, or by varying the gain only slightly, when the level of the audio signal is relatively large (so signals that are already loud are not further boosted). In this manner a signal with a dynamic range of say 40 dB may be compressed to a range of 20 dB to insure that all of the signal is louder than the noise. U.S. Pat. No. 4,641,344 describes one system using this approach in which the amount of compression applied is determined by the ambient noise. This system is disadvantageous because it compresses the audio signal in response to noise even when the entire audio signal could be comfortably perceived without any compression.
U.S. Pat. No. 4,944,018 teaches another system for compensating for the dynamic range problem. This system compresses the audio signal so that the smallest expected audio signal is boosted so that it is comparable to the noise level. This system is disadvantageous in that the amount of compression applied by the system, which is determined by the noise level, is also determinative of the maximum volume level, so the maximum volume level fluctuates with the noise level.
U.S. Pat. No. 5,208,866 teaches another system for compensating for the apparent volume problem and the dynamic range problem. This system generates a control signal by calculating the difference between the audio signal level and the noise level inside the vehicle, and uses this control signal to adjust the volume and the amount of compression. This system is disadvantageous because the amount of compression it provides fluctuates with changes in the audio signal level.
In general, systems designed to compensate for either the apparent volume, uneven masking, or dynamic range problems require some way of estimating the power level of the ambient noise. Automotive audio reproduction systems often use a microphone disposed inside the passenger compartment of the automobile to measure the noise, however, the use of microphones has several disadvantages. Since the microphone is generally sensitive to all sounds in the automobile (including the acoustic signals generated by the audio reproduction system) and not just the noise, it is generally necessary to filter the signal generated by the microphone to generate a signal representative of the noise. One method of filtering the microphone output signal is to use a low pass filter to remove all frequencies in which the acoustic signal may be present. This, however, generates a signal that is representative only of the sub-audio low-frequency noise and not the actual masking noise in the higher frequencies. Another method of filtering the microphone output signal is to determine the contribution made to the microphone output signal by the acoustic signal (which was generated by the audio reproduction system) and to subtract this contribution from the microphone output signal and thereby obtain a signal representative of the noise. This method is disadvantageous because it requires additional circuitry and also requires knowledge of the transfer function from the speakers of the audio reproduction system to the microphone. This transfer function is difficult to determine and moreover it varies dynamically, e.g., with changes in the number of passengers in the vehicle and changes in vehicle parameters such as window position. Further, methods of filtering the microphone output signal generally do not permit measurement of ambient noise in the frequency range which is occupied by human speech, and therefore do not describe fluctuations in the frequency distribution of the noise. Finally, microphones are relatively expensive.
Another method of measuring the noise in an automobile is to construct a filter which transforms the output signal of a sensor, such as a speedometer, into a signal representative of the noise. This method is generally less expensive than those which use microphones, however this method also has several disadvantages. In general, there is no determinative relationship between a single sensor output signal and the noise since the noise depends on several factors such as vehicle speed, wind velocity, and the smoothness of the road surface. Systems of this type, such as the one described in U.S. Pat. No. 4,558,460, therefore generally provide only a relatively inaccurate measurement of the noise.
It is therefore an object of the invention to provide an improved system for compensating for the apparent volume, uneven masking, and dynamic range problems.
It is a further object of the invention to provide an improved system for estimating the ambient noise in a noisy environment.
Other objects and advantages of the present invention will become apparent upon consideration of the appended drawings and description thereof.